204 THE 



possibilities of development into such complex attached organ- 

 isms as the higher spore plants and seed plants were determined 

 by those changes in the habits of algae by which the motile 

 periods in the life history became reproductive stages, and the 

 quiescent conditions came to be the conspicuous part of the life 

 history as the vegetative plant body was gradually developed. 



232. Summary of the reproductive organs and processes of 

 the green algae. Zoospores, also called swarm spores, are ciliate 

 asexual cells (generally two- or four-ciliate), and are developed 

 as a rule numerously (in some forms singly, or in twos, fours, etc.) 

 in a mother cell called a sporangium, or zoosporangium. 



Gametes are sexual cells. The simplest forms are ciliate and 

 have the same form and structure (morphology) as the zoospores, 

 to which they are related. These in the process of sexual evo- 

 lution became differentiated into eggs and sperms. Gametes are 

 developed in cells called gametangia. 



Eggs are never ciliated, and are consequently nonmotile. 

 They are generally large cells, with abundant chromatophores 

 and food material. Eggs are formed hi cells called oogonia. 



Sperms, frequently called antherozoids by botanists, are always 

 ciliated in the green algse and are very actively motile. They 

 are smaller than zoospores and colorless, or almost colorless. 

 Sperms are developed in cells called antheridia, or a group of 

 such cells is frequently termed an antheridium. 



Isogamy is the sexual condition in which the gametes are 

 similar in form and structure ; that is, they have the same mor- 

 phology. They may differ in size. The sexually formed cell is 

 called a zygospore. 



Heterogamy is the sexual condition in which the gametes are 

 different hi form and structure, as the sperm and egg, and there- 

 fore have a different morphology. They are always very unlike 

 in size, but this does not make heterogamy, because morphology 

 does not deal with size but with form. The egg is fertilized by 

 the fusion and entrance of a sperm and thus becomes a fertilized 

 egg, or, if it develops a protective cell wall, an oospore,. 



